FIG. 8 is a circuit block diagram of a conventional in-vehicle power supply device 500 that has a stop-idling function and is mounted to a vehicle. A positive electrode of storage battery 1 is connected to input terminal 2a of power supply circuit 2 through fuse 3 while output terminal 2b of power supply circuit 2 is connected to load 4. In power supply circuit 2, boost converter 5, connection-assist diode 6, and switch 7 are arranged in parallel, and are connected to input terminal 2a and output terminal 2b. An anode of connection-assist diode 6 is connected to input terminal 2a of power supply circuit 2 while a cathode of connection-assist diode 6 is connected to output terminal 2b of power supply circuit 2.
When a vehicle is restarted from a stop-idling state, boost converter 5 of power supply circuit 2 boosts a voltage of storage battery 1, thereby restarting the vehicle stably. Switch 7 opens only when boost converter 5 performs a boost-up operation. In this operation, control unit 8 mounted to the vehicle controls boost converter 5 and switch 7. Further, in order not to supply the boosted voltage to input terminal 2a while boost converter 5 performs the boost-up operation, connection-assist diode 6 of power supply circuit 2 is provided. Besides, in order to supply electric power to load 4 from storage battery 1 even if switch 7 is damaged for some reason to continuously open, connection-assist diode 6 is provided.
That is, storage battery 1 can supply electric power to load 4 as long as either connection-assist diode 6 or switch 7 is in a normal state.
In the case that connection-assist diode 6 and switch 7 both open, electric power is supplied to boost converter 5 from storage battery 1 continuously, so that boost converter 5 may be damaged. To prevent this, control unit 8 detects a voltage at output terminal 2b of power supply circuit 2. Control unit 8 constantly monitors whether switch 7 opens or is closed in response to an instruction from control unit 8.
As for the monitoring method, firstly, control unit 8 previously detects a voltage at output terminal 2b of power supply circuit 2 while switch 7 is securely closed, and then stores the detected voltage.
If electric power is supplied to output terminal 2b of power supply circuit 2 from storage battery 1 through connection-assist diode 6 in the case that switch 7 fails to be closed in response to an instruction of control unit 8, a voltage drops due to connection-assist diode 6. For this reason, the voltage at output terminal 2b output through connection-assist diode 6 has a lower value than the voltage stored previously. Control unit 8 monitors a state of switch 7 based on the difference between these voltage values.
When determining that switch 7 has abnormality, control unit 8 sends this situation to the outside immediately, and thus boost converter 5 is intentionally avoided from being damaged.
A control unit similar to the above-mentioned device is disclosed in, e.g. PTL 1.